Conventionally known as one security device is an MW sensor wherein microwaves are transmitted toward protected area(s), and, in the event that person(s) is or are present within protected area(s), wave(s) reflected from such person(s) (microwave(s) modulated due to the Doppler effect) are received and person(s) (intruder(s)) is or are detected (e.g., Japanese Patent Application Publication Kokai No. H7-37176 (1995)).
Moreover, also known as one type of MW sensor is a device which employs a plurality of microwaves of different frequency and which is constituted so as to permit measurement of distance(s) to object(s). In this type of sensor, microwaves of, for example, two different frequencies are transmitted toward protected area(s), and phase difference(s) between two IF signals based on respective reflected waves is or are detected. Correlation(s) exist between or among such phase difference(s) and distance(s) to target(s) (person(s) and/or other such object(s) intended to be detected), phase difference(s) tending to increase with increasing distance(s) to target(s). In other words, distance(s) to target(s) can be measured by calculating such phase difference(s). Below, operations for detection of phase difference(s) between/among IF signals in this type of sensor are described. Taking the case where IF signals based on waves produced by reflection of microwaves of two different frequencies are sinusoidal waves IFout1, IFout2 (having a phase difference corresponding to distance to target) as shown at FIG. 3 (a), rectangular waves A, B derived from these IF signals might respectively be as shown at FIG. 3 (b). It will, moreover, be possible to measure the distance to the target by detecting the phase difference between these rectangular waves A, B (the phase difference Δt at the rising edge portion of the rectangular waves in the drawing).
However, where this type of sensor is installed indoors, measurement errors may occur due to the influence of microwaves reflected by ceiling surface(s), wall surface(s), and/or floor surface(s). More specifically, for targets present at the extreme near side of the sensor range (e.g., on the order of 0 to 2 m) or targets present at locations relatively distant from the sensor (e.g., locations 8 m or more from the sensor), such measurement errors are small. However, for targets present at locations other than the foregoing (e.g., locations on the order of 3 to 7 m from the sensor), it is possible that measurement error will be large. The reason this is the case is described below.
Referring to FIG. 4 (a), description is first made with respect to a situation where the target is present at the extreme near side of the sensor range. Receiving antenna(s) for this type of sensor “a” are chosen such that directionality with respect to reception of reflected waves received from the front (reflected waves in horizontal direction(s) which are directed toward the left in the drawing) is high; and conversely, such that directionality with respect to reception of reflected waves from the side(s) and from regions thereabove and therebelow (e.g., the reflected wave indicated by the alternating long and short chain line in the drawing) is low. For this reason, if target “b” is present at the extreme near side of the range of sensor “a”, microwaves from sensor “a” will directly irradiate target “b”, these will also be directly reflected therefrom onto sensor “a” (the irradiated wave and reflected wave indicated by solid lines in the drawing; reflected waves received in such fashion being hereinafter referred to as normally reflected waves), and the received reflected wave signal level will be high with respect thereto. In contrast thereto, the received signal level will be extremely low for signals received at sensor “a” after reflection by ceiling surface “c”, floor surface “d”, and so forth as indicated by alternating long and short chain lines in the drawing. For this reason, because the reflected waves forming the IF signal for measurement of distance to target “b” are for the most part made up of the normally reflected waves indicated by the solid lines in the drawing, there is almost no occurrence of measurement error.
Referring to FIG. 4 (b), description is next made with respect to a situation where target “b” is present at a location relatively distant from sensor “a”. In such a situation, where irradiated waves are reflected by ceiling surface “c” or the like as indicated by the dashed line in the drawing, or where reflected waves are reflected by floor surface “d” or the like as indicated by the alternating long and short chain line in the drawing, the path taken by microwaves will be longer than the path taken by normally reflected waves (the path indicated by solid lines in the drawing). However, because the distance to target “b” is large, it is possible to hold the difference between the foregoing paths to a value at or below on the order of 10 percent as a fraction of the distance to this target “b”. For example, taking a case where target “b” is present at a distance of 10 m from sensor “a”, even if the difference between the foregoing paths is as much as 1 m it will still be possible to hold the error to values at or below 10 percent. For this reason, errors due to reflection of microwaves at ceiling surface “c” and floor surface “d” have almost no effect.
In contrast thereto, where, as shown at FIG. 4 (c), target “b” is located neither at the extreme near side of the range of sensor “a” nor at a far distance from sensor “a”, reflected waves received at sensor “a” after reflection by floor surface “d” and so forth as indicated by alternating long and short chain lines in the drawing will be received from angles near the front of the antenna, and the received signal level of such signals will be relatively high. Furthermore, where irradiated waves are reflected by ceiling surface “c” or the like as indicated by the dashed line in the drawing or where reflected waves are reflected by floor surface “d” or the like as indicated by the alternating long and short chain line in the drawing, the path taken by microwaves is longer than the path taken by normally reflected waves. Because the distance from sensor “a” to target “b” is also relatively short it is possible that the difference between the foregoing paths could be large (e.g., on the order of 50 percent) as a fraction of the distance from this sensor “a” to target “b”. In other words, reflected waves traveling via paths much longer than paths of normally reflected waves are received at sensor “a” with relatively high received signal levels. For this reason, IF signals being formed from waves representing superposition of the foregoing normally reflected waves and reflected waves traveling via paths longer than paths of such normally reflected waves, measurement errors arising due to the influence of the latter, i.e., reflected waves (reflected waves traveling via paths longer than paths of normally reflected waves) will increase, greatly impairing reliability of sensor “a”.
Such problems are not limited to situations in which MW sensor(s) is or are installed indoors, but will also occur in similar fashion where MW sensor(s) is or are installed outdoors if object(s) constituting obstacle(s) is or are present within area(s) intended to be protected and microwaves are reflected by such object(s).